Tough choice for you because of all of the specs to consider but an easy choice for me because I've considered both and more. Not that I can afford either or just one Axis lol. There's better choices to be honest, from TC even, like if you used four of the LM 10's or 12's, they have a better response, not needing nearly as much space to get flat @ a lower frequency - 1500W RMS and you can buy like 8 LM's for the price of two Ultra 18's. 8 LM 10's would handle 12k and they'd need 16 cubes sealed and you'd get +120 more inches of cone area, not that that matters that much.
No they wouldn't be peaky like an SPL sub, especially if you tuned that low they would still hit the highs well though, more so the Axis because of more cone area. Did you read the memo? The LMS Ultra is the god-send of subwoofers so get those. These are low-distortion SQ and street-bass subs.
Just because LMS Ultra's have low distortion doesn't mean they can't over-excurt when playing below the tuning frequency and it'll sound good doing it so you wouldn't notice, wouldn't you want to?
If you're at all serious about low end SPL (20-30 Hz) and want to keep your drivers alive than seal them! Even the 15's.
The TC LMS Ultra? (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=293-666)
Use this to model the frequency response of a sub in a sealed box:
http://www.carstereo.com/help/Articles.cfm?id=30
TC LMS Ultra sealed (below .4 = bad, over 1 = bad, .707 = flat). They recommend using no less than 2.6 or 2.8 cubes sealed.
If you want no less than about -8dB to -10dB @ 20 Hz than go with no less than 5 and up to 7.5 (useless to go over) cubes sealed each. I would go with a .5 response in 6.5 to 6.56 (QTC exactly .5 in 6.56 cubes) cubes @ FC 31.14 Hz (6.5 cubes) or 31.06 Hz (close enough for me, what's 1 Hz?).
You can fit about 7-7.5 cubes each so that's perfect for the lows that you want. Jamaica would feel it.
My dream setup for SQ & lows is two Ultra 18's on 10k in 10 cubes sealed in my trunk. I listen to a lot of chopped, daz dillinger type beats and jamaican (damien/x-clan) stuff. You jerk.